As one of the most important multiple access methods in a wireless network system with a high data rate in the future, Orthogonal Frequency Division Multiple Access (OFDMA), also called Orthogonal Frequency Division Multiple (OFDM) for short, has been written into a Worldwide Interoperability for Microwave Access (WiMAX) protocol and a Long Term Evolution (LTE) protocol as specifications, and has been determined as an access technology in two candidate standards of the International Mobile Telecommunications-Advanced (IMT-Advanced): the LTE-Advanced promoted by the third generation partnership project (3GPP) and the 802.16m promoted by the Institute of Electrical and Electronics Engineers (IEEE).
Generally, the OFDM technology divides a channel into several orthogonal sub-channels, and converts a high speed data signal into parallel low speed sub-data-streams which are modulated to respective sub-channels for transmission. In this way, multiple users in the same cell can perform transmission simultaneously on different sub-carriers, thereby decreasing the interference in the cell significantly. However, when users of the edge of adjacent cells are allocated with the same frequency band, inter-cell interference (ICI) has great effect on the performance of the system. To solve the problem, considering the attenuation characteristic of electromagnetic waves during propagation in the space, after one frequency is used in a certain area, the power has attenuated greatly at a place far away from the area above, and the interference has decreased to an acceptable degree, the frequency can be used again, thus the concept of frequency reuse is put forward. To achieve the purpose of avoiding ICI to the greatest extent, the frequency reuse solution usually is implemented by configuring a proper frequency reuse factor (FRF). If the FRF is too small, it seems that very high frequency spectrum efficiency can be reached, but it also means the probability that the users at the boundary of the adjacent cells use the same frequency is increased, and the ICI caused between the users at the edge of the cells is intensified, thus affecting the quality of service at the edge of the cells. On the other hand, if the FRF is too high, the frequency spectrum efficiency of the system is decreased, in this way it is hard to meet high quality and high speed service demands for the fourth generation communication (4G) system. Therefore, it is very essential to select a proper reuse factor according to the deployment scenario of a cell.
A traditional frequency reuse solution includes frequency reuse with an FRF of 3, fractional frequency reuse and soft frequency reuse, etc. The frequency reuse solution with the FRF of 3 is to divide frequency resources into three equal sub-bandwidths, and adjacent cells use three different sub-bandwidths respectively to avoid the interference between the cells caused by the same frequency. The fractional frequency reuse solution is that the same frequency is used inside all the cells, high FRF is adopted at the cell edge and different frequencies are used at the edge areas of adjacent cells with an aim of reducing the ICI effectively and increasing the frequency spectrum efficiency. The soft frequency reuse solution is that a power factor is taken into consideration based on the fractional frequency reuse and the frequency spectrum is no longer divided into several parts mechanically but with the use degree of the frequency spectrum specified by the power, which enables the FRF make a smooth transition from 1 to N so as to obtain a greater bandwidth and frequency spectrum efficiency. Meanwhile, if sectorization, time, power, load and other factors are taken into consideration, more variations of the frequency reuse solution can be obtained.
The IMT-Advanced, started by the International Telecommunications Union (ITU) with the aim of meeting the global mobile communication requirements in the future 10 to 15 years, features higher data rate, greater system capacity, more flexible and extensive service and application, and more powerful ability to support new services. To meet these requirements, almost all the candidate standards of the IMT-Advanced introduce Relay transmission as a key technology into the development process of the standards. At present, there are three relay standards that have been or are being specified in IEEE 802.16j, IEEE 802.16m and 3GPP LTE-A Release 10. Relaying refers to that, by adding intermediate nodes between a base station (BS) and a mobile station (MS), signals sent by the BS or the MS are re-generated, amplified and then forwarded to the MS or the BS, so as to improve the quality and reliability of the signal transmission. The main role of the relay technology is to extend the coverage area of the cell, provide service signals for the area with severe shadow fading in the cell and dead spots of coverage, provide the coverage for the hot region and indoor coverage, etc. The 3GPP divides a relay station (RS) into Type1 Relay and Type2 Relay according to whether the RS has an independent cell identity (ID). Type1 Relay, having an independent cell ID, can transmit a reference signal and a synchronization signal of the Type1 Relay, and has independent Hybrid Automatic Repeat Request (HARQ) feedback, etc. At this time, the RS is equivalent to a Release 8 BS, and the MS can tell apart an RS and a BS. However, Type2 Relay does not have an independent cell ID, and the MS can not identify the RS. The RS can transmit a service channel, but can not transmit a common pilot signal (CRS) and a Physical Downlink Control Channel (PDCCH), etc. because the RS has no independent cell ID. In addition, according to whether a relay link and an access link share the same frequency resources or not, the RS can also be divided into an in-band relay and an out-band relay.
However, regardless of the types of the RS and the relay deployment scenario, the frequency planning and frequency reuse of the BS and the RS shall be considered to solve the ICI problem, so as to extend the coverage area of the cell, improve the system capacity, peak rate and other performance. Compared with a cellular network without relay deployment, in a cellular network with relay deployment, in addition that a BS performs direct communication with a mobile station MSc within the coverage of the BS, it is also required that the BS performs indirect communication with a mobile station MSr at the cell edge through multi-hop relay, namely the first hop link BS→RS and the second hop link RS→MSr. Therefore, the frequency reuse solution of a relay system shall be more flexible compared to that of a traditional OFDM cellular cell. Similar to the frequency reuse solution of a traditional cellular network, the frequency reuse solution of the cell in the relay scenario can be derived, such as the soft frequency reuse and the fractional frequency reuse of two-hop relay. However, the main purpose of most of the traditional frequency reuse solutions is to eliminate the interference between cells to the greatest extent, or further to take the frequency spectrum efficiency into consideration, but the problems of user fairness and load balancing are ignored to some degree.